There are a number of pumps on the market that work with a principle of having a tube-shaped membrane inside a tubular housing that can be alternatively compressed and expanded by for example pressurized air in order to move a volume of liquid through the pump housing. Examples of such pumps are disclosed in EP 1122435, JP 01240777 and JP03149373.
The pump disclosed in EP 1122435 is quite complicated in its design with an elongated flexible tube that is placed inside a pipe and when inflated will act on material in the pipe. In order to push material along the length of the pipe, the tube is arranged with sections of increasing wall thickness along its length, whereby the section with the thinnest wall will be inflated first and then successively the sections with increasing wall thickness will be inflated in sequence.
The two other documents disclose a similar solution with an inner, flexible hose. Pressurised medium between elongated pipe sections and the flexible hose is controlled such that the hose will be inflated in a wave-like manner along the length of the sections, creating a movement of material inside the pipe sections in one transport direction.
A further pump solution utilizing the above mentioned principle is disclosed in document SE 520389C2. The pump comprises an elongated tube having a somewhat conical shape. Inside the tube a flexible hose is arranged along the length of the tube and attached at the ends. The tube is arranged with an air inlet and an air outlet. During use, pulses of compressed air are forced into the air inlet whereby the tube is radially compressed such that an annular space is created. The annular space moves along the length of the tube towards the air outlet, bringing liquid inside the hose with it. When the space reaches the air outlet, the compression terminates. At the same time, successive spaces are created by pulses of air entering through the air inlet.
A drawback with the solution according to SE 520389C2 is that the pulsed creation of the spaces in the pump and the successive and pulsed movement of volumes of liquid cause pulsed forces in the whole pump system that have negative effects on the performance of the pump. It would be a benefit if the effects of these forces where reduced in order to increase the capacity of the pump.